In a semiconductor wafer, it is general that after forming a circuit on its front face, in order to adjust a wafer thickness, the wafer back face side is subjected to grinding processing. In grinding processing of the wafer back face, a surface protective sheet for protecting the circuit, which is called a back grind tape, is stuck onto the front face of the wafer.
In recent years, following advancement and complication of semiconductor products, a circuit formed on the wafer front face is liable to be influenced by static electricity. For that reason, in order to prevent releasing electrification or the like on releasing, the back grind tape has been being demanded to have an antistatic performance. In addition, following ultrathinning of a wafer in the recent years, in order to prevent wafer warpage or the like after grinding, the back grind tape is demanded to have high stress relaxation characteristics.
Then, as disclosed in PTL 1, there has hitherto been taken a technology that by not only forming a support film using a urethane-based oligomer but also blending the support film with a metal salt antistatic agent, a pressure sensitive adhesive tape is given an antistatic performance and high stress relaxation characteristics.
But, in the pressure sensitive adhesive tape of PTL 1, if the addition amount of the metal salt antistatic agent is increased, change in physical properties is caused, for example, the film rigidity is worsened, and a metal ion derived from the antistatic agent moves into a pressure sensitive adhesive and further moves into a front face circuit of a semiconductor wafer that is an adherend, thus providing a concern of causing a fault. For that reason, the pressure sensitive adhesive tape disclosed in PTL 1 involves such a problem that a high antistatic performance is hardly given.
In addition, PTLs 2 and 3 disclose that a substrate of a pressure sensitive adhesive tape for semiconductor processing is provided with, as a different layer from a support film, an antistatic layer having an antistatic agent, such as a conductive polymer, a quaternary amine salt monomer, etc., blended therein. But, likewise PTL 1, those using such a conductive polymer or quaternary amine salt monomer involve such a problem that a high antistatic performance is hardly given.
Meanwhile, PTL 4 describes that a conductive tin oxide powder having excellent conductivity can be utilized as an antistatic agent. When a metal filler disclosed in PTL 4 is, for example, used in place of the antistatic agent disclosed in PTLs 2 and 3, it is expected that the antistatic performance of a back grind tape is improved.